The present invention relates in general to a method and apparatus for digitizing an analog waveform and more particularly to a waveform digitizer providing hardware averaging and autocalibration of waveform samples.
Sampling oscilloscopes were developed more than twenty years ago to respond to small, fast-changing signals to which conventional oscilloscopes could not respond due to limited bandwidth or risetime characteristics. Sampling is a now well-known technique wherein a signal path is gated for an extremely short period of time to pass the substantially instantaneous amplitude value (voltage sample) of an electrical signal during that period. Each sample taken in this manner is processed by electronic circuits and displayed as a dot on a cathode-ray tube (CRT) display, the dot being horizontally positioned according to sample time and vertically positioned according to sample magnitude. Since a large number of samples are required to accurately reconstruct a waveform, sampling is most practical when the electrical signal is repetitive in nature since in most cases it is impossible to acquire all of the needed samples during a single event or single cycle of the signal. Indeed, one of the advantages of sampling is that at least one sample can be acquired from each of a large number of cycles, and a representative waveform may be reconstructed and displayed therefrom.
Sampling modes are typified in accordance with the timing method used. Sequential sampling is a mode in which the display is comprised of a very orderly series of equally spaced dots. Random sampling is a mode in which successive dots may occur at what appear to be random horizontal positions because the sampling timing and signal triggering are unrelated, although it must be pointed out that with random sampling the reconstructed waveform is defined because the dots are inserted into the display at substantially correct time positions.
Prior art sampling systems, referred to herein as classic samplers, include a high-speed sampling gate, a sampling capacitor, a memory gate, a memory capacitor, associated amplifiers including a sampling preamplifier and a memory amplifier, and a feedback attenuator from the memory amplifier output to the sampling preamplifier input. A sampling loop is formed having a forward gain from input to output and a feedback attenuation factor from output to input to establish a loop gain which is ideally unity. The purpose of the feedback amplifier is to place a voltage on the sampling capacitor which is equal (for unity loop gain) to the last sample taken as an estimator of the next sample to be taken. With each sample, the memory output repeatedly attempts to reduce to zero the voltage existing between the input and output of the sampling gate. If the input voltage is the same each time it is sampled, the feedback matches it, reducing the difference and the size of each amplified step to practically zero. The ability of a sampling oscilloscope to display correctly the voltage change between any two successive samples is known as a dot transient response, and good dot transient response requires unity loop gain.
There are situations in which it is desirable to have a loop gain which is not equal to one. For example, in a process known as smoothing, i.e., reducing the effect of random noise or jitter in the display, the loop gain is set to some value less than one. However, smoothing results in a degraded dot transient response, since the estimator placed on the sampling capacitor is derived from the previous sample and may be different from the new sample about to be taken. The viewed reconstructed waveform has the appearance of having been passed through a low-pass filter, and while noise is reduced on the displayed waveform, the input signal is distorted. Moreover, signal noise is not completely eliminated, but rather is distributed along the displayed waveform.